The present invention relates generally to electric and hybrid-electric vehicles, and more particularly to canceling leakage current during charging of a hybrid-electric vehicle.
The unintended circulation of current, or leakage current, is a pervasive problem in electric and hybrid-electric vehicles. The large number of high voltage devices and wiring introduce the problem of leakage currents that may ultimately cause safety, electromagnetic compatibility and charging concerns with the vehicles. There are many causes of leakage current, and the effects include safety concerns that are of the utmost importance in addressing.
The United States National Electric Code (NEC) dictates the requirement that electric and hybrid-electric vehicle chassis be referenced to earth through a safety ground whenever the vehicle is being charged. This requirement is in effect to guarantee that leakage currents are safely shunted to earth. Because the NEC also requires that electric utility service outlets be protected by devices that measure the current imbalance between the conductors, the potential exists for these devices to disconnect power to the vehicle if the current imbalance exceeds the code value, i.e. 5 milliamps for level 1 charge. These devices protect persons in the event of conduction of current through an unintended path that exceeds 5 milliamps, because the power is disconnected thereby preventing a hazardous shock. The NEC establishes specific requirements for an electric or hybrid-electric vehicle that requires a device to disconnect power at approximately 20 milliamps for level 2 charging. If there is any fault in the safety ground, the power is disconnected.
AC leakage components introduce an imbalance between two primary conductors in the vehicle being charged that can activate the ground fault circuit. This is called nuisance activation. In the event of nuisance activation, the power to the charger is disconnected, and the vehicle will not be properly charged. The result of an uncharged vehicle is severe customer dissatisfaction.
Another cause of leakage current is related to electromagnetic compatibility. High frequency current components on the vehicle being charged can conduct back onto an ac line and cause harmful interference with receiving equipment, such as televisions or radios. In order to control the conducted noise within limits regulated by the United States Federal Communication Commission, filters are used to attenuate the conducted noise. However, the filters contribute to leakage current.
In an attempt to prevent leakage currents, it has been proposed to galvanically isolate high voltage from the charge line. This approach is accomplished by coupling a transformer to the charge line. However, the transformer is very large and very heavy. High frequency transformers that are smaller and lighter may be used, but add complexity and cost due to the high cost of the transformers and the switching devices. Another alternative is to disconnect high voltage components during charging, virtually eliminating some of the major leakage paths. However, this approach is not always practical. Often cabin preheat and pre-cool functions that precondition the vehicle temperature just before driving are present on vehicles to offer significant battery energy savings. These features require heaters and air conditioners be active even during charging demanding the high voltage components remain active. Therefore, there is a trade-off between a reduction in leakage current and an efficient use of valuable battery energy.
It is clear that there are many causes to the problem of leakage current, but there are too few solutions. What is needed is an effective device for eliminating leakage current that does not add significant cost and/or weight to an electric or hybrid-electric vehicle.
It is an object of the present invention to provide reliable charging of an electric or hybrid-electric vehicle. It is another object of the present invention to eliminate the need for galvanic isolation of a charging device.
It is a further object of the present invention to cancel leakage current during charging of an electric or hybrid-electric vehicle. It is still a further object of the present invention to introduce a counter leakage potential in order to cancel leakage current.
In carrying out the above objects and other objects and features of the present invention, a leakage current cancellation device is provided that uses line voltages that are out of phase with each other to produce a counter leakage potential that cancels leakage current. The device is located on board an electric or hybrid-electric vehicle between the incoming charge line and the vehicle charger.
According to the present invention, a differential transformer senses the leakage current. Each of two primary line voltage conductors passes through the differential transformer. A current imbalance in the transformer produces a potential across the transformer sense winding that is amplified. Because the imbalance is a measure of the net leakage, the potential across the sense winding is representative of the leakage current. When the net leakage exceeds a predetermined value, a reverse potential is induced to cancel the leakage current. The leakage current may be continuously monitored to compensate for time varying transients.
In another embodiment of the present invention, the potential amplifier is replaced with a microprocessor that senses the leakage on each conductor independently and adjusts the induced reverse potentials individually.
Other objects and advantages of the present invention will become apparent upon reading the following detailed description and appended claims, and upon reference to the accompanying drawings.